Generally, semiconductor devices are used in a variety of electronic applications, such as computers, cellular phones, personal computing devices, and many other applications. Home, industrial, and automotive devices that in the past comprised only mechanical components now have electronic parts that require semiconductor devices, for example.
Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semiconductive, and insulating materials that are patterned and etched to form integrated circuits (IC's). There may be a plurality of transistors, memory devices, switches, conductive lines, diodes, capacitors, logic circuits, and other electronic components formed on a single die or chip.
Semiconductor devices typically include one or more metallization layers that are usually formed in the upper material layers in a back end of the line (BEOL) of the fabrication process. The metallization layers include conductive lines comprised of conductive materials such as aluminum (Al), copper (Cu), and other metals, which are separated from other conductive lines by an insulating material such as silicon dioxide (SiO2).
Electromigration is a phenomenon that occurs in conductive materials that can become problematic in semiconductor devices having small conductive features. Electromigration is the transport of material caused by the gradual movement of atoms in a conductive material, due to a momentum transfer between conducting electrons and diffusing metal atoms. Electromigration occurs when electrons transfer momentum to atoms, which cause the atoms to move from their original positions.
Over time, the movement of atoms from their original positions due to electromigration can cause a break or gap in the conductive material, preventing or hindering the flow of electrical current. In narrow interconnect conductors, such as conductive lines linking transistors and other components in integrated circuits, this is known as a void or internal failure open circuit. Electromigration can also cause the atoms of a conductor to pile up and drift toward other nearby conductors, creating an unintended electrical connection known as a hillock or whisker failure, or a short circuit. Both voids and hillock failures can lead to a malfunction of an integrated circuit.
Electromigration is particularly a problem in applications where high direct current densities are used. With increasing miniaturization, the probability of failure due to electromigration increases in semiconductor devices, because smaller conductive lines have increased power density and current density. Electromigration can lead to the eventual loss of one or more connections, and to the intermittent failure and/or eventually permanent failure of an entire circuit.
Electromigration is an inherent limitation for very large scale integration (VLSI) interconnect systems, because electromigration reliability issues limit current density increase in ever-shrinking VLSI circuitry. Electromigration performance testing is consequently a very important factor during the development of technology and also in the manufacturing process of semiconductor devices. One key cost issue facing semiconductor device designers is the acceleration of electromigration testing times, which currently may range from several hours to several days, for example.
Temperature and current density are two factors involved in the acceleration of electromigration testing times. Both factors are limited: increased temperature can initiate other non-electromigration degradation mechanisms, while increased current density tends to cause temperature inhomogeneities in test structures, deteriorating or inhibiting the electromigration assessment. Temperature inhomogeneities are especially pronounced in test structures involving both conductive lines and vias or contacts, for example. Via and contact issues may be independent from conductive line issues and have become critical issues for Cu and AlCu interconnect systems, for example.
Thus, what are needed in the art are improved structures and methods for testing electromigration in semiconductor devices.